@Captain:
You seem to be a bit behind, or you skipped somewhere. With that in mind, you may wanna stop reading now, or spoilers will be abound.
Human transmutation was already explained in depth, in that it's impossible. Due to all the minute physical details within each and every person, a person, literally, cannot be transmuted back to life. This was further demonstrated when the failed Trisha was dug up; it had different hair color and other details than Trisha.
In the FMA manga world, it's physically impossible to recreate someone. There were a few chapters dedicated to this
No, I understand that much. But Father obviously has been able to bend the rules of traditional Alchemy. Plus, he's not humanoid. Scar is able to decompose the Homunculus because they have the same body structure as us, but his attempt to deconstruct father failed. However, unlike Ed and Al, his and Mei Chang's alchemy worked in the presence of father, because of its origins being from Ling. And hence, that's why they're heading to the far north to discover Scar's brother's hidden research.
The thing is, while they were underground fighting father, alchemy didn't work on the surface, either. Meaning either Father's circle of influence caused it, or machinery even further beneath them was jamming Amestris Alchemy within a certain radius. If it was father, then that would only raise further questions.
I guess I didn't word what I'm trying to say correctly. Basically, I'm looking for an explanation on HOW Father knew that what Ed and Al